In recent years, different types of solid hot melt or phase change inks have been proposed and used as an alternative to the older and well developed liquid inks for printheads of thermal ink jet printers. As is generally well known in the art, these solid hot melt inks have been developed to undergo a phase change between solid and liquid when heated up to a predetermined elevated temperature. These solid phase change inks have proven to be an attractive alternative to liquid inks for certain types of printing applications because of the recent improvements in print quality on a wide variety of papers, improved waterfastness, and a potential for a higher throughput.
The delivery temperature of the hot melt ink in an ink jet printhead is normally in the range of 60.degree.-200.degree. C., with the precise value within this range depending upon the ink formulation. To maintain desirable physical properties for ink drop ejection and drop volume consistency, the ink delivery temperature must be controlled within a narrow range around a nominal value, typically plus or minus 5.degree. C. or better. The regulation of this temperature poses challenges for accurate, low cost, and reliable means suitable for use with color ink jet print cartridges using hot melt inks.
Several approaches have been proposed in the prior art for providing this temperature control, and examples of these approaches may be found in U.S. Pat. No. 4,490,731 issued to Vaught, U.S. Pat. No. 4,667,206 issued to DeYoung, U.S. Pat. No. 4,631,557 issued to Cooke, U.S. Pat. No. 4,607,266 issued to DeBonte, and U.S. Pat. No. 4,593,292 issued to Lewis, all incorporated herein by reference. However, generally speaking, these prior approaches have not proven entirely satisfactory because, among other reasons, they have employed temperature sensors such as thermocouples, thermistors, and temperature sensitive resistors and associated circuitry having a relatively high degree of cost and complexity associated therewith. In addition, these prior art approaches have the further disadvantage of requiring dedicated interconnect pads, carriage printed circuit board circuitry and trailing cables for passing low power analog signals to these temperature sensors which are located adjacent to the supply of hot melt ink within the print cartridge.